In studies of the effects of serotonin, ascorbic acid, prostaglandin E1 and ionophore A-23187 on human leukocytes, it was found that agonists that increase cyclic GMP enhance chemotaxis whereas those that increase cyclic AMP inhibit it. Several lines of cultured lymphocytic leukemia cells have been characterized in terms of cyclic GMP content, responsiveness to serotonin and guanylate cyclase activity. Soluble and solubilized particulate guanylate cyclases from renal cortex have been partially purified and compared.